I haven't seen this exact solution explained anywhere, so posting for others who might feel stuck like I did.
As nicely explained by E73RN I Erj in the other guide, to obtain the perk(s) to craft legendary items, you need to have either level 20 Intelligence or level 18 Technical ability. But, you only get 71 attribute points to spend, and I did not wisely spend enough of those 71 points on Intelligence or Technical ability, so I was locked out from advancing either of those attributes far enough to unlock either of the needed perks.
However, you fortunately do have the ability to craft one legendary item - the legendary System Reset quickhack.
In the side job Spellbound, you need to successfully datamine the Book of Spells (sequence is BD E9 BD 55 1C FF BD) to gain the crafting spec for the legendary System Reset. Completing the side job Kold Mirage will reward you with an epic System Reset which is needed to craft the legendary System Reset.
Before starting the point of no return main job Nocturne Op55N1, craft the legendary System Reset quickhack twice. Craft the first one by using the epic System Reset rewarded for completing Kold Mirage. Sell the legendary System Reset you just crafted to any vendor (you can't craft the legendary System Reset again if you already possess one). Then go to the Netrunner who has an epic System Reset for sale in the northern part of Japantown (just to the east of the Skyline & Salinas fast travel point). Buy it, then craft the second legendary System Reset.
Once you complete the final mission and return to free roam, visit the Netrunner in northern Japantown again. He should have another epic System Reset for sale. If you still have the legendary System Reset in your inventory, sell it, then buy epic System Reset, craft another legendary System Reset, and then the achievement should unlock!
Important: Make sure you craft two legendary System Resets BEFORE starting Nocturne Op55N1!